1. Field of the Invention
The present invention relates to an error signal detection apparatus for an optical recording/reproducing system, and more particularly, to an error signal detection apparatus for an optical recording/reproducing system, which can detect a tangential tilt error signal and/or a radial tilt error signal with respect to a relative tilt between an objective lens and a recording medium, and/or a defocus error signal, using a main light beam used for recording an information signal on or reproducing an information signal from the recording medium.
2. Description of the Related Art
Optical pickups record an information signal on or reproduce the information signal from a recording medium, such as an optical disc seated on a turntable and rotating, while scanning the recording medium in the radial direction. However, if the rotating optical disc is tilted with respect to the optical axis, due to bending of the optical disc itself or due to an error in loading the disc, degradation of a recording/reproduction signal could occur.
When an optical pickup adopts a light source, which emits a shorter wavelength of light and an objective lens having a high numerical aperture (NA) to increase recording density, comma aberration caused by a tilt of the optical disc increases, thereby further degrading the recording/reproduction signal. Comma aberration increases as the tilt of the optical disc increases because optical aberration is proportional to λ/(NA)3.
In an optical recording/reproducing system for high-density recording and reproduction in a recording medium, such as a digital versatile disc (DVD) and/or next generation DVD series (so-called high definition (HD)-DVD), there is a need for a tilt error signal detection apparatus for correcting a record/reproduction signal according to a relative tilt between the recording medium and the objective lens.
To accurately detect a radial tilt error signal, there is a need for the tilt error signal detection apparatus in the optical recording/reproducing system to detect a radial tilt error signal without being influenced by a tangential tilt, a defocus, or a detrack. To accurately detect a tangential tilt error signal, there is a need for the tilt error signal detection apparatus for in the optical recording/reproduction system to detect a tangential tilt error signal without being affected by the radial tilt, the defocus, or the detrack.
On the other hand, as shown in FIG. 1, a light beam reflected by a recording medium 10, after being focused as a light spot on the recording medium 10, is diffracted into a 0th order diffracted beam and ±1st order diffracted beams by, for example, pits (P) formed on the recording medium 10. Thus, a photodetector 9 in the optical pickup to detect an information signal receives the 0th order diffracted beam and the ±1st order diffracted beams. For the 0th order diffracted beam and ±1st order diffracted beams received by the photodetector 9, the 0th order diffracted beam overlaps each of the ±1st order diffracted beams that do not overlap each other. Detection signals from the overlapping portions between the 0th order diffracted beam and each of the ±1st order diffracted beams, and a detection signal from a pure 0th order diffracted beam have different phase characteristics. That is, the phase characteristics of individual light beam portions reflected/diffracted by the recording medium 10 differ from one to another. The phase characteristics of the detection signals vary according to a degree of tangential and/or radial tilt.
In a conventional error signal detection apparatus for detecting a tangential or radial tilt, a light beam reflected by the recording medium 10 is received as four divided light beams by the photodetector 9, which is divided into four sections. The four divided light beams are photoelectrically converted, separately, and the detection signals from the four divided light beams are summed or subtracted to detect a tangential or radial tilt error signal. Thus, the original phase characteristic according to position in the light beam, which is reflected/diffracted from the recording medium 10, cannot be reflected sufficiently in detecting a tangential or radial tilt error signal. As a result, the tangential or radial tilt error signal cannot be accurately detected.
On the other hand, when defocus occurs in recording data in the next generation DVD series recording medium for high-density recording, a method for compensating for defocus is needed because the high-density recording medium is greatly affected by the defocus compared to a conventional recording medium, which uses a red laser beam. In other words, when a short-wavelength light source, for example, having a blue wavelength of 420 nm or less, and an objective lens having an NA of 0.6 or more are used for high-density recording, a defocus margin becomes smaller due to a small focus depth of an incident light beam. As a result, even if a small degree of defocus occurs in the recording data, the problem of the defocus can be serious for the high-density recording medium.
As well known, when recording is performed on the DVD series recording medium, defocus may be controlled with a variation of about 230 nm by using a 650-nm light beam and an objective lens having an NA of 0.6. However, when recording is performed on the next generation DVD series recording medium using a blue light and an objective lens having, for example, an NA of 0.85, there is a need to accurately control the defocus to be within the range of tens of nanometers.
To precisely control the defocus to be within the range of tens of nanometers for data recording on the next generation DVD series recording medium, there is a need to detect a defocus error signal without being affected by detrack, and a relative tangential and radial tilt between the objective lens and the recording medium.